Gas turbine engines (GTEs) convert potential energy associated with air and fuel into energy, primarily in the form of mechanical rotation and heat. A conventional GTE may include a compressor assembly, a combustor assembly, and a turbine assembly. During operation, air is drawn into and compressed within the compressor assembly. The combustor assembly receives the compressed air, supplies fuel thereto, and ignites and combusts the compressed fuel-air mixture. The combustion products are supplied to the turbine assembly and expanded to cause a turbine rotor to rotate, thereby producing rotational energy. The turbine assembly is typically coupled to the compressor assembly, which uses some rotational energy developed by the turbine to compress air. Further, a gas turbine engine may include a recuperator (or heat exchanger) to heat compressed air before it enters the combustor by recovering heat from the turbine exhaust. Even with such a heat exchange, the flow of compressed air reduces the temperature of the combustor assembly.
Fuel for combustion is provided by means of fuel injectors that provide both a main fuel stream and a pilot fuel stream. The main fuel stream comprises a leaner fuel-air mixture, and the pilot fuel stream comprises a richer fuel-air mixture. The main fuel stream burns more efficiently than the pilot fuel stream, producing a lower temperature flame with relatively low NOx emissions. The richer fuel-air mixture, directed to the combustor as the pilot fuel stream, bums at a higher temperature and serves to stabilize the combustion process at the cost of slightly increased NOx emissions.
Under certain conditions, a portion of the compressed air exiting the compressor assembly may be bled off before the compressed air is sent to the combustor section (and typically before such compressed air is heated by the recuperator). The compressed air bypassing the combustion process is known as bleed air, and the compressed air that is used in the combustion process is known as combustion air. Bleed air may be released by way of a bleed air valve (or bleed valve) controlled by the GTE's control system. There are various reasons for bleeding air before combustion including, for example, stabilizing combustion and controlling engine performance.
Rotation power generated by a GTE may be used to drive a load. For example, a GTE may be used to drive a power generator, and such a generator may generate large amounts (on the order of megawatts) of electrical energy, which may, for example, be provided to an electrical power grid (also known as a utility grid). A GTE driving a full (or otherwise substantial) load is considered to be in an “on-load” condition; a GTE driving no load (or a negligible load) is considered to be in an “off-load” condition; and a GTE driving a load substantially less than its full load is considered to be in a reduced load condition. Under certain circumstances, such as, if the utility grid goes offline or if the circuit breaker for the power generator is triggered, the load being driven by the GTE may drop substantially in a short period of time to an off-load or reduced load condition, perhaps to as low as 500 kilowatts or even 0 megawatts. This is known as an off-load transient. If the rotational power of the GTE is not reduced commensurate with the drop in load, the GTE may go into an overspeed state, creating potentially hazardous conditions that may damage the GTE and its surroundings.
In order to compensate for an off-load transient—and subsequent off-load or reduced load conditions—the total fuel supply to the combustor may be reduced and the ratio of pilot fuel to total fuel may be greatly increased. For example, when a GTE is driving a full load, the ratio of pilot fuel to total fuel may be approximately 2%. And, during an off-load transient, the pilot fuel ratio may increase to approximately 40%. This pilot fuel ratio increase is accomplished by both reducing the flow of main fuel into the combustor and by increasing the flow of pilot fuel. In order to avoid an overspeed state, the reduction in total fuel supply must occur rapidly in response to the commencement of an off-load transient. And in order to avoid combustion instability, the ratio change must occur rapidly in response to the commencement of an off-load transient. A high pilot fuel ratio may permit the GTE to continue to run with stable condition during the reduced load condition, thereby preventing a shut-down of the GTE. A shut-down of the GTE can be particularly problematic where a GTE located in a power plant is disconnected from the utility grid (for example, due to power transmission line failure). In such circumstances, a shut-down of the GTE could lead to a complete power supply black-out in the power plant in which the GTE is located.
The various embodiments of the present disclosure are directed toward overcoming one or more of the problems set forth above and/or other problems of the prior art.